1. Field of the Invention
The present invention relates to a torque distribution control apparatus for a four-wheel drive vehicle and a four-wheel drive vehicle having the torque distribution control apparatus.
2. Description of the Related Art
Japanese Patent No. 3603569 discloses that driving force output from an engine is transmitted to a transmission via a clutch and driving force output from the transmission is transmitted to driving wheels via a power transmission system such as a differential gear. When a quick acceleration request is made by the driver, the engine speed or the engine output torque is limited so as to prevent the differential gear from damage.
When front wheels serving as main driving wheels slip and a rotation speed difference is produced between the front wheels and rear wheels serving as sub-driving wheels at the start of a four-wheel drive vehicle, driving-force distribution clutches in the rear differential gear are engaged to distribute the driving torque to the rear wheels. Thereby, the driving torque to be distributed to the front wheels is reduced to suppress the slip, and traction is produced in the rear wheels so that the vehicle can start moving. If an actual driving torque produced in the rear wheels by engagement of the torque distribution clutches excessively increases, the load on the rear differential gear increases, and this may reduce durability.
By limiting the engine speed or the engine output torque, as in the invention of Japanese Patent No. 3603569, the driving torque to be distributed to the rear wheels can be reduced and the rear differential gear can be protected. In this case, unfortunately, the total traction of the front wheels and the rear wheels decreases, and this reduces starting performance of the vehicle.